Basicity 
The basic components that caused PAC changing is OH ion, a measure of OH ions index in poly aluminum chloride called basicity (Basicity, B for short), usually the basicity is defined as OH and Al equivalent percentage of the poly aluminum chloride molecules. (〔OH〕/〔Al〕×100(%))。
1 Basicity determination
  1.1  Method summary 
Adding quantitative hydrochloric acid solution in the sample, with potassium fluoride masking aluminum ion, titrate with Standard NaOH solution.
Reagents and materials

1.1.1.1 hydrochloric acid(GB/T 622)：C(HCl )about 0.5mol/L solution
1.1.1.2 sodium hydroxide(GB/T 629)：C(NaOH)=0.5000mol/L standard titration solution
1.1.1.3phenolphthalein(GB/T 10729)：10g /L ethanol solution
1.1.1.4 potassium fluoride(GB/T 1271)：500g/L solution, stored in plastic bottles
1.2 Analysis steps
Remove about 0.6g solid samples, accurate to 0.0002g. move 20-30ml water into 250ml conical flask, add 25mL of hydrochloric acid with a pipette, covered with a watch glass, heat in a boiling water bath for 10 min, cooling to room temperature. Plus 25mL potassium fluoride solution, shaken up. Add 5 drops of phenolphthalein indicator solution, titrate with sodium hydroxide standard titration solution to the solution was reddish immediately is the end. At the same time, using Carbon dioxide-free distilled water for blank test.

1.3 The representation of the analysis results

Calculate by the basicity (X2) expressed as a percentage as follows:

In the formula,

V0--- The volume of the standard titration of sodium hydroxide solution consumption for blank test. ml

V--- The volume of the standard titration of sodium hydroxide solution consumption by sample solution, ml

C---Actual concentration of the standard titration of sodium hydroxide solution, mol/l

m--- The quality of the sample, g

X1---Article 4.1 determination of alumina content, %;

0.01699--- The quality of alumina expressed in grams as1.00ml standard titration of sodium hydroxide solution.

1.4 Allowable error
Take arithmetic mean of parallel determination results as determination results, the absolute difference of parallel determination results is not greater than 2.0%.

The effect on coagulation by basicity
Summary: using the coagulation and sedimentation test method to study the relationship between PAC basicity and coagulation effect, the result shows that, in the basicity range of 0~92%, the Coagulation effect increased with the basicity increasing, but the change slowing down when the basicity is more than 80%. Compared with the standards and products at home and abroad, the upper limit of basicity has a larger expanding.

The fundamental difference between PAC and traditional inorganic coagulant is traditional inorganic coagulant is low molecular crystal salt, but the structure of the poly-aluminum chloride composed by shape changing multi-hydroxy complex and polymer, Thus the poly aluminum chloride showed many specific coagulation function different from traditional coagulants. The basic components that caused PAC changing is OH ion, a measure of OH ions index in poly aluminum chloride called basicity (Basicity, B for short), usually the basicity is defined as OH and Al equivalent percentage of the poly aluminum chloride molecules. ［1、2］：B=［OH］/［Al］×100。PAC can be understood as the hydrolysate between normal salt of AlCl3 and alkali Al3. Usually, PAC basicity lies in the range of 16.7% to 83.3%, the standard basicity in Japan lies in the range of45%~65%.

In addition to OH,AL equivalent ratio basicity, there are OH,AL mole ratio basicity, but not widely used. Tang hongxiao propose first at home and abroad to use formation function F replace basicity B as basic characteristic parameter ［3］, named as hydrolysis degree B, and he considered the conception of hydrolysis degree B can express

The structure composition of PAC accurately, and can reflect the dynamic process of hydrolysis.

According to the author's work practice, the conception of OH,AL equivalent ratio basicity have good guidance in production of feed and production application, analysis and the calculation is simple, it has better practical value. the raw materials and process of PAC production in China is different from other countries in the world, thus the basicity as the main quality indicators have larger differences from foreign countries.

State Construction Committee set provisional quality indicators of PAC in 1973 as ［4］, defined basicity to be  50%～80%, expanding the range of Japanese standard basicity for the first time; in 1981 the standard of Sichuan defined basicity to be  45%～85% drafted by the author; national standard GB15892—1995 defined basicity to be  50%～85% based on this. The PAC standards basicity indicators at home and abroad are in tabale 1.

Table 1:The PAC standards basicity indicators at home and abroad

In the late 1980s, Chinese original calcium aluminate raw material and corresponding adjustment method production process, made the PAC process greatly simplified, reduced the investment and the production cost greatly, the product basicity achieved more than 90%, higher than the level of the same period abroad introduced by the document, raised the PAC production practice and theory to a new height. Thus ,how to improve the basicity of PAC is a research direction of science and technology at home and abroad.
1 Experimental section
1.1 Test using PAC
Liquid PAC, calculate by basicity, a series products made by the adjustment process with the acid-soluble calcium aluminate in industrial enamel reactor; Solid poly aluminum chloride come from Jiangsu Yixing, Taicang and Henan Sanmenxia and other production plants and foreign products.
1.2 Test instrument
Coagulation test,  adopt ZR4 - 6 intelligent automatic coagulation test mixer of Shenzhen zhongrun company; turbidimetric analysis, adopt 2100P type turbidimeter of American HACH company;  PH determination, adopt type 520 pH gauge of American ORILON company.
1.3 Experiment conditions.

Hybrid G value ： 500～1000s-1， time: 40s; Flocculation G value : 10～100s-1, time: 10min, GT:*104, sedimentation time:10min.
2 Results and discussion
2.1 Different basicity liquid PAC coagulation effect①The same dosage
Different basicity liquid PAC coagulation effect showed in table 2
Table 2 Different basicity liquid PAC coagulation effect

Table 2 shows that, to all the raw water, adding same dosage, purified water residual turbidity decreases with increasing basicity.
The basicity and purifying water turbidity residual curve

thus it can be seen, the purified water residual turbidity decreases with increasing basicity. In the range of 0~80% of basicity, the amplitude reduction is large; In the range of 80%～92%, the amplitude reduction is slowing, the basicity is 88%, close to the residual turbidity of 92%.

②different dosage
Different basicity liquid PAC coagulation effect of different dosage showed in table 3
According to table 3, the variation of different basicity liquid PAC residual turbidity of different dosage is basically the same as adding same dosage.
Table 3 Different basicity liquid PAC coagulation effect of different dosage

With different basicity of PAC treat the raw water turbidity residual to 5NTU dosage, called equivalent dosage. table 3 indicated that, use basicity 65% equivalent dosage is 100% as reference, B=45% adding quantity ratio is 106%, B=92% adding quantity ratio is 106%,that means, B=92% dosage is lower 47.9% than B=45%.
2.2 different basicity of solid PAC coagulation effect
Comparative test of different basicity of solid PAC coagulation effect shows that, the purified water residual turbidity decreases with increasing basicity of different basicity of solid PAC, when B in the range of 64%～90.6%, and dosage is 3.5mg/L; but when dosage ＜3mg/L the purified water residual turbidity has slight rising trend. Compared the basicity of 94.1% and 90.6%, the former’s residual turbidity rise in the round, but still close to the basicity of 77.6%.

2.3 the composition and coagulation effect of different products at home and abroad
Experiments with different domestic and foreign production technology, different basicity of solid, liquid poly aluminium chloride product components showed in table 4.
Table 4  Experiments PAC product components at home and abroad

Coagulation effect comparison in table 5
Table 5  Coagulation effect of PAC at home and abroad

No matter solid or liquid PAC, adding same dosage, Coagulation effect of Chinese product is better than foreign product. generally speaking, using same process technology, the PAC basicity is higher, the raw material consumption and cost is lower; in water treatment process adding same dosage consume less water alkalinity. Using appropriate technology, the liquid PAC stationary phase can last more than 1 year, Not less than the traditional craft products. Liquid products in different drying process, the basicity improve about 2%～4%
When the basicity improved more than 85%, it requiring an accurate feeding calculation and process control, the difficulties of solid-liquid separation also increased, therefore we should adopt advanced technology.
3 conclusion
①Basicity of PAC products is the most important quality indexes, it is also the most important production control parameters.
②OH, Al equivalence ratio basicity has  better correlation between coagulation effect and controlling conditions of producing, therefore, basicity concept still has practical value.
③The liquid PAC basicity index , considering of the feasibility of production control, production cost and the coagulation effect, recommended standard values ranged from 45% to 90%.

④The solid PAC basicity index, based on the value of liquid , considering of the basicity promotion on the drying process, recommended standard values ranged from 45% to 95%.

⑤Improve the basicity of PAC product, can greatly improve the production and use of economic benefits.improve the basicity from 65% to 92%, the production cost of raw materials can be reduced by 20%, The use cost can be re